1. Technical Field
The present invention relates to wireless communication and, more particularly, to a method of generating a reference signal in a wireless communication system.
2. Related Art
In general, a sequence is used for various channels and signals in a wireless communication system. The sequence in the wireless communication system needs to satisfy the following characteristics:
(1) Good correlation characteristics to provide high detection performance,
(2) Low CM (Cubic Metric) to enhance efficiency of a power amplifier,
(3) Generation of a large number of sequences to transmit a large amount of information or to facilitate cell planning,
(4) Being able to be generated in a closed form in order to reduce a capacity of a memory for the sequence.
A downlink synchronization channel is used to perform time and frequency synchronization between a base station and a user equipment and to perform cell searching. A downlink synchronization signal, namely, a sequence, is transmitted on the downlink synchronization channel, and synchronization is performed through a correlation operation with the received downlink synchronization signal. A physical cell ID can be identified through the downlink synchronization channel. Because a unique cell ID should be identified, as the number of available sequences is increased, it is advantageous in terms of cell planning.
An uplink synchronization channel is used to perform time and frequency synchronization and to perform access for a network registration, a scheduling request, or the like. A sequence is transmitted on the uplink synchronization channel, and each corresponding sequence is recognized as a single opportunity. Upon detecting a sequence, the base station can recognize through which opportunity the user equipment has transmitted the uplink synchronization channel. In addition, through the detected sequence, a timing tracking, a residual frequency offset, or the like, may be estimated. As the number of opportunities is increases, probability of collision between user equipments can be reduced. Thus, a larger number of sequences is advantageous in terms of cell planning. The uplink synchronization channel is called a random access channel (RACH) or a ranging channel depending on a system.
A sequence can be used as control information transmitted on a control channel. This means the control information such as an ACK (Acknowledgement)/NACK (Negative-Acknowledgement) signal, a CQI (Channel Quality Indicator), etc. can be mapped to the sequence. The larger number of available sequences is advantageous to transmit various control information.
A scrambling code is used to provide randomization or peak-to-average power ratio (PAPR) reduction. In terms of cell planning, a larger number of sequences are advantageous to be used for scrambling codes.
When several users are multiplexed in a single channel through code division multiplexing (CDM), a sequence may be used to guarantee orthogonality among users. A multiplexing capacity is related to the number of available sequences.
A reference signal is used by a receiver to estimate a fading channel and/or is used to demodulate data. Further, the reference signal is used to obtain synchronization when the user equipment awakes from a time/frequency tracking or in sleep mode. In this manner, the reference signal is utilized variably. The reference signal uses a sequence, and the larger number of sequences is advantageous in terms of cell planning. The reference signal is also called as pilot.
There are two types of uplink reference signals: a demodulation reference signal and a sounding reference signal. The demodulation reference signal is used for channel estimation for data demodulation, and the sounding reference signal is used for user scheduling. In particular, the uplink reference signal is transmitted by a user equipment with a limited battery capacity, so PAPR or CM characteristics of the sequences used for the uplink reference signal are critical. In addition, in order to lower the cost of the user equipment, it is necessary to reduce the amount of the memory required for generating sequences.